In a common-rail injection system, the activation period of at least one injector which is necessary for the desired injection quantity is ascertained as a function of a desired injection quantity and a measured fuel pressure in the rail (the so-called rail pressure) via an activation period characteristics map. In addition, the setpoint value of the rail pressure is predefined in such a way that it is continuously adjustable via a rail pressure characteristics map as a function of the desired main injection quantity and the rotational speed of the internal combustion engine.
The activation period characteristics map corresponds to an average or mean-valued injector, so that only an exactly mean-valued injector will precisely inject the desired fuel quantity during the activation in the case of the activation period ascertained on the basis of the activation period characteristics map.
In order to adapt or compensate for manufacturing tolerances of injectors, versus an above-mentioned mean-valued injector, as well as, for example, age-related changes in the injection behavior over their service lives, the different approaches or functions described in the following are known to be available.
In order to adapt quantity variances in the new condition of an injector, a so-called injector fuel-quantity compensation is carried out. In this case, the actually injected fuel quantity is individually measured for each injector at different points of the activation period characteristics map and their deviation from a setpoint quantity of a mean-valued injector is determined. The measured values of the deviation are, for example, alphanumerically coded and this code is attached to the injector. During the manufacture of an internal combustion engine, this code is read into a control device of the internal combustion engine and decoded. Quantity deviation values present at discrete check points or base points are interpolated or extrapolated across the entire activation period characteristics map. The above-mentioned deviations of the injection quantity are therefore known for each injector or cylinder of the internal combustion engine and may thus be adapted or compensated for. An injector fuel-quantity compensation, however, does not allow for the compensation for the above-mentioned changes over the service life.
With the aid of a learning function of the so-called “zero fuel compensation” (ZFC) which is known per se, a quantity drift may be adapted over the service life in a very limited characteristics map area of an above-mentioned activation period characteristics map. This function includes certain learning period areas, exclusively time intervals in the coasting mode of the internal combustion engine being used in which a quantity intended by the driver equals zero. In an operating state of this type, a certain learning value is set for the rail pressure and the injector of a cylinder is in this case activated at a very short activation period. This activation period is varied until a predetermined injection quantity may be inferred from the rotational speed progression of the internal combustion engine. In this way, it is possible to retrace and adapt or compensate for the change in the activation period which results over the service life and which is necessary for the injection of the predetermined injection quantity. The disadvantage is that the learning mode and the application of a learned activation period is limited to very small injection quantities so that if an injection quantity >0 is desired by the driver, the learning cycle must be interrupted immediately in most cases and a transition into the normal driving operation must take place.
Another approach to adapting an injection system depends on the basis for compensating for torque inequalities between individual cylinders of the internal combustion engine (so-called “fuel balance control”=FBC), which mainly originates in the air system or in the mechanical components of the internal combustion engine. In this case, this compensation takes place with the aid of pure controllers.
Moreover, sensor systems for detecting the injection start and the injection end of a fuel injection or for measuring the combustion chamber pressure are already being developed today. It will thus be possible to control the injection quantity significantly more accurately than before, it being necessary, however, to stationarily set the working point for the control for a preferably long period of time. Working points of this type, however, change constantly and rapidly in a motor vehicle so that it will also be necessary in systems of this type to ascertain preferably accurate information about the general deviation of the behavior of an individual injector from that of an above-mentioned mean-valued injector in order to use this information in the form of a pilot control of the activation period or a variable associated with the latter.